Not the death of optimism, just the winning of the NASA bureaucracy in keeping the organization size by pushing the "cheaper" space shuttle.

With the benefit of hindsight, flying into space with the X-15's successors would have happened if there was not a need to beat the Soviets quickly with rockets.

Also throwing away the Saturn rockets after around 20 launches was just silly.

Correct on all counts. Turning those remaining Saturn Vs into lawn ornaments was criminal.

I believe that the apotheosis of human achievement was likely around 1974 or thereabouts.

The rest is just incrementalism which is more than cancelled out by serious backsliding all the way around.